Pixar's movies (Monsters, Inc., Finding Nemo, and The Incredibles) have relied heavily on a sophisticated mathematical technique called dynamic simulation to shape the final look and behavior of the movies' main characters. From the beginning, however, it has been set in stone that the use of dynamic simulation could not interfere in Pixar's traditional creative process. Senior animation scientist David Baraff gives a candid behind-the-scenes look at the core physical simulation technologies employed in Pixar's recent movies, describes the balancing of creative and technical needs due to simulation, and reveals the difficult effects that were easy, and the simple shots that were hard.